All crude oil contains impurities which contribute to corrosion, heat exchanger fouling, furnace coking, catalyst deactivation and product degradation in refinery and other processes. These contaminants are broadly classified as salts, bottom sediment and water (BS+W), solids, and metals. The amounts of these impurities vary depending upon the particular crude. Generally, crude oil salt content ranges between about 3-200 pounds per 1,000 barrels (ptb.).
Brines present in crude include predominantly sodium chloride with lesser amounts of magnesium chloride and calcium chloride being present. Chloride salts are the source of highly corrosive HCl, which is severely damaging to refinery tower trays and other equipment. Additionally, carbonate and sulfate salts may be present in the crude in sufficient quantities to promote crude preheat exchanger scaling.
Solids other than salts are equally harmful. For example, sand, clay, volcanic ash, drilling muds, rust, iron sulfide, metal and scale may be present and can cause fouling, plugging, abrasion, erosion and residual product contamination. As a contributor to waste and pollution, sediment stabilizes emulsions in the form of oil-wetted solids, and can carry significant quantities of oil into the waste recovery systems.
Metals in crude may be inorganic or organometallic compounds which consist of hydrocarbon combinations with arsenic, vanadium, nickel, copper and iron. These materials promote fouling and can cause catalyst poisoning in subsequent refinery processes, such as catalytic cracking methods, and they may also contaminate finished products. The majority of the metals carry as bottoms in refinery processes. When the bottoms are fed, for example, to coker units, contamination of the end-product coke is most undesirable. For example, in the production of high grade electrodes from coke, iron contamination of the coke can lead to electrode degradation and failure in processes, such as those used in the chlor-alkali industry.
Desalting is, as the name implies, adapted to remove primarily inorganic salts from the crude prior to refining. The desalting step is provided by adding and mixing with the crude a few volume percentages of fresh water to contact the brine and salt.
In crude oil desalting, a water in oil (w/o) emulsion is intentionally formed with the water admitted being on the order of about 4-10 volume % based on the crude oil. Water is added to the crude and mixed intimately to transfer impurities in the crude to the water phase. Separation of the phases occurs due to coalescence of the small water droplets into progressively larger droplets and eventual gravity separation of the oil and underlying water phase.
Demulsification agents are added, usually upstream from the desalter, to help in providing maximum mixing of the oil and water phases in the desalter. Known demulsifying agents include water soluble salts, Twitchell reagents, sulfonated glycerides, sulfonated oils, acetylated caster oils, ethoxylated phenol formaldehyde resins, a variety of polyester materials, and many other commercially available compounds.
Desalters are also commonly provided with electrodes to impart an electrical field in the desalter. This serves to polarize the dispersed water molecules. The so-formed dipole molecules exert an attractive force between oppositely charged poles with the increased attractive force increasing the speed of water droplet coalescence by from ten to one hundred fold. The water droplets also move quickly in the electrical field, thus promoting random collisions that further enhance coalescence.
Upon separation of the phases from the W/O emulsion, the crude is commonly drawn off the top of the desalter and sent to the fractionator tower in crude units or other refinery processes. The water phase containing water soluble metal salt compounds and sediment is discharged as effluent.
Desalters are often employed in tandem arrangement to improve salt removal efficacy. Commonly, in such designs, crude oil from the resolved emulsion in the first desalter is used as crude feed to the downstream second desalter. Wash water is added to this crude separated from the emulsion in the first desalter with water phase bottoms effluent from the second desalter being fed back as make up water, mixed with the crude fed to the first desalter.